The invention relates generally to a method and apparatus for generating coherent near-millimeter wave radiation. More particularly, the invention relates to a method and apparatus for improving the efficiency of a Smith-Purcell free-electron laser, or orotron.
In an orotron, a ribbon-like electron beam is directed over a metallic reflecting diffraction grating embedded in a mirror which, along with another mirror, forms an open resonator. This resonator reflects the radiation emitted by the electron beam back into the beam, causing the electrons therein to bunch. If the proper conditions of synchronism between the electron beam velocity and the phase velocity of an evanescent wave traveling along the grating are met, coherent radiation will result.
In orotrons and orotron-like devices developed in the U.S.S.R. and Japan, a convergent electron gun has been used with a beam focusing magnetic field to produce ribbon-like electron beam having an electron beam density in the range of 20 to 30 amperes per square centimeter. However, until now, all electron guns for orotrons have utilized conventional thermionic cathodes which require cathode heater sources.
Prior to the present invention, the effects of mutual electronic repulsion in the electron beam in an orotron (space charge effects) had not been considered in theoretical models for orotrons developed in the U.S.S.R. and in this country. Space-charge effects appeared to have negligible effect on the efficiency of an orotron, since published test data indicated that the orotron output was almost a linear function of the beam current. For example, in an article by F. S. Rusin and G. D. Bogomolov entitled "The Orotron, An Electronic Device With An Open Resonator And A Reflecting Grating", published in Izvestiya VUZ. Radiofizika, Vol. 2, No. 5, pp. 758-762, 1968, the results of tests performed on an orotron to determine the orotron power as a function of beam current were published. These tests indicated that the prior output of the orotron depended almost linearly on the beam current over a wide range.